1. Field of the Invention
The present invention relates to a boot covering a constant velocity joint that is indispensably used for a joint of a drive shaft used in a front drive vehicle in order to prevent water or dust from entering into a joint portion of the constant velocity joint.
The present application is based on Japanese Patent Application No. Hei. 11-239408, which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, in a joint portion of a constant velocity joint, a smooth revolution at a large angle is maintained by being covered by a bellowed boot to prevent water and dust from entering therein. The constant velocity joint boot comprises a first cylindrical portion of a large diameter being held by a joint outer race or the like, a second cylindrical portion of a smaller diameter than the first cylindrical portion and being held by a shaft, and a bellowed portion in the shape of a generally truncated triangular pyramid for integrally connecting the first cylindrical portion and the second cylindrical portion. When in use, since a bellowed portion deforms with angle variations between the joint outer race or the like and the shaft, the joint portion may be reliably sealed by the boot even when the angle is significantly large.
The bellowed portion of the conventional boot constant velocity joint is formed with crests and roots continuing alternately and respective crests and roots are generally formed in geometrical similarity.
In a conventional constant velocity joint boot, the outer diameters of the crest portions are relatively large in order to grease the way to accommodate variations in the angle defined between the central axis of a joint outer race or the like and the central axis of a shaft and reduce stresses generated at the bellowed portion. However, in recent years, as weight reduction of vehicles has been asked for, miniaturization of the constant velocity joint boots is also asked for in conjunction with miniaturization of vehicles.
Miniaturization of constant velocity joints boots inevitably requires decrease in outer diameter of the bellowed portion. Since reduction of outer diameter of the bellowed portion shortens the length of membrane forming crest portion correspondingly, it is required to increase the depth of the root portions in order to maintain the amount of displacement of the bellowed portion and stresses generated thereby as they were before miniaturization. However, as shown in FIG. 5, simply increasing the depth of the root portion may result in that the second root portion 12 may be jammed between the inboard joint outer race 100 and the shaft 101 when the angle defined by the central axis of the inboard joint outer race 100 or the like and the central axis of the shaft 101 increases. When the root portion is jammed in such a manner, a strong force is applied on the root portion, which may damage the root portion and thus impair the sealing property thereof. Therefore, the boot has to be replaced often, which turns into problem of short life.
In view of above described circumstances, it is an object of the present invention to present a constant velocity joint boot in which a jam of the root portion may be prevented even when the angle between a pair of axes of rotation defined by a joint outer race or the like and a shaft is significantly large and the dimensions thereof are reduced.
The constant velocity joint boot of the present invention for solving above described problem comprises a first cylindrical portion, a second cylindrical portion being spaced from and coaxial to the first cylindrical portion and having a smaller diameter than the first cylindrical portion, a bellowed portion having a shape of generally truncated triangular pyramid for integrally connecting the first cylindrical portion and the second cylindrical portion, characterized in that the bellowed portion comprises a plurality of crest portions and root portions arranged in such a manner as, from the side of the first cylindrical portion, a first root portion, a first crest portion, a second root portion, a second crest portion, a third root portion, a third crest portion, etc, continuously and alternately, that the outer diameter of the first crest portion is almost the same as that of the first cylindrical portion, and that in the cross section taken through a plane including the central axis, the length of the line connecting the top of the first crest portion through the second root portion to the top of the second crest portion is almost the same as that of the line connecting the top of the second crest portion through the third root portion to the top of the third crest portion, and that the depth of the second root portion (h) and the depth of the third root portion (H) satisfy the relational expression H/1.3xe2x89xa6hxe2x89xa6H/1.1.
In the constant velocity joint boot of the present invention, the bellowed portion comprises a plurality of crest portions and root portions arranged in such a manner as, from the side of the first cylindrical portion, a first root portion, a first crest portion, a second root portion, a second crest portion, a third root portion, etc, continuously and alternately, and the outer diameter of the first crest portion is almost the same as that of the first cylindrical portion. The outer diameter of the crest portions become smaller gradually from the second crest portion on, forming a shape of generally truncated triangular pyramid. Therefore, in the constant velocity joint boot of the present invention, since a line connecting the top of each crest portion is an almost straight line and the outer diameter of each crest portion of the bellowed portion is smaller than that of the first cylindrical portion, it is very much miniaturized in size.
In the constant velocity joint boot of the present invention, in the cross section taken through a plane including the central axis, the length of the line connecting the top of the first crest portion through the second root portion to the second crest portion is almost the same as that of the line connecting the top of the second crest portion through the third root portion to the top of the third crest portion. In other words, the length of the membrane extending between the first crest portion and the second crest portion is almost the same as that extending between the second crest portion and the third crest portion. Therefore, a sufficient amount of deformation may be ensured within the range from the first crest portion through the second root portion, the second crest portion, the third root portion, and the third crest portion where a large amount of deformation is expected as a constant velocity joint boot. It is also preferable to make the length of the membrane extending between any adjacent crest portions the same length respectively. If there is a portion where the length of membrane is relatively short, there may be a recognized disadvantage that the fatigue resisting property is degraded because the portion is subject to a significant stress. Provided that the length of membrane in the vicinity of the crest portion of the smallest diameter located adjacent the second cylindrical portion is excluded since the amount of deformation is quite small around this portion.
In the boot for a constant velocity of the present invention, the depth of the second root portion (h) and the depth of the third root portion (H) satisfy the relational expression:
H/1.3xe2x89xa6hxe2x89xa6H/1.1
By defining the depth of the second root portion that is most vulnerable to a jam when being deformed within the range shown above, a jam of the second root portion may be prevented even when the angle defined by a pair of axes of rotation is increased to about 50 degrees. For example, in the case where h=H/1.2 is satisfied, a jam at the second root portion can be prevented even when the angle defined by a pair of axes of rotation is increased up to about 52 degrees. In the case where the depth of the second root portion (h) is below H/1.3, it will be difficult to ensure a sufficient amount of deformation because the length of membrane of the second root portion is too short, and thus leading to degradation of the fatigue resisting property thereof. When the depth (h) of the second root portion is over H/1.1, a jam may occur at the second root portion.
The length of the membrane from the first crest portion through the second root portion to the second crest portion is almost the same as that from the second crest portion through the third root portion to the third crest portion. However, since the depth h of the second root portion is smaller than the depth H of the third root portion, as a next logical step, the angle at the second root portion is larger than that at the third root portion.
Since preventing a jam at the second root portion prevents a jam at the third root portion and thereafter from occurring, the constant velocity joint boot of the present invention does not suffer from a jam even when the angle defined by a pair of axes of rotation is significantly large such as 50 degrees or so, and thus degradation of sealing property caused by damage thereof is prevented with reliability. Therefore, the boot may have long lasting qualities.
As regards the structure other than those described above, the constant velocity joint boot of the present invention may have the same structure as conventional boot. As a material thereof, thermoplastic elastomer or the like which may be formed generally by blow molding may be employed. The number of the crest portions and the root portions and the thickness thereof may be selected according to the application.
Features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.